Mat Chapter 23

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Mat Chapter 23
  COMPOSITE FLOORS - I COMPOSITE FLOORS - I 1.0INTRODUCTION Traditional steel - concrete composite floors consist of rolled or built-up structural steel beams and cast in-situ concrete floors connected together using shear connectors in sucha manner that the !ould act monolithicall Fig#$%# The principal merit of steel-concretecomposite construction lies in the utilisation of the compressi&e strength of concrete slabsin con'unction !ith steel beams( in order to enhance the strength and stiffness of the steelgirder#More recentl( composite floors using profiled sheet dec)ing ha&e become &er popular in the *est for high rise office buildings# Composite dec) slabs are particularlcompetiti&e !here the concrete floor has to be completed +uic)l and !here mediumle&el of fire protection to steel !or) is sufficient# ,o!e&er( composite slabs !ith profileddec)ing are unsuitable !hen there is hea& concentrated loading or dnamic loading instructures such as bridges# The alternati&e composite floor in such cases consists of reinforced or pre-stressed slab o&er steel beams connected together to act monolithicall#  tpical composite floor sstem using profiled sheets is sho!n in Fig#.# There is presentl no Indian standard co&ering the design of composite floor sstems using profiled sheeting#/esigning a reinforced concrete slab or pre-stressed concrete slab in compositeconstruction is not different from an con&entional R#C# or pre-stressed structures0 hence(this is not discussed an further here# In this chapter( concrete floors using profiled dec)sare treated in depth# The structural beha&iour of these floors is similar to a reinforcedconcrete slab( !ith the steel sheeting acting as the tension reinforcement# The mainstructural and other benefits of using composite floors !ith profiled steel dec)ing are1 ã Sa&ings in steel !eight are tpicall 30%  to 50%  o&er non-composite construction ã 2reater stiffness of composite beams results in shallo!er depths for the same span#,ence lo!er store heights are ade+uate resulting in sa&ings in cladding costs(reduction in !ind loading and sa&ings in foundation costs# ã Faster rate of construction# The steel dec)ing performs a number of roles( such as1 ã It supports loads during construction and acts as a !or)ing platform ã It de&elops ade+uate composite action !ith concrete to resist the imposed loading ã It transfers in-plane loading b diaphragm action to &ertical bracing or shear !alls ã It stabilises the compression flanges of the beams against lateral buc)ling( untilconcrete hardens# ã It reduces the &olume of concrete in tension 3one ã It distributes shrin)age strains( thus pre&enting serious crac)ing of concrete# Version II23 - 1 23  COMPOSITE FLOORS - I 4 Copright reser&ed Version II23 - 2 Fig. 1 Steel beam bonded to concrete slab with shear connectors Section A-A showing dimples Fig. 2 Composite floor system using profiled sheets  A A Profiled sheet   COMPOSITE FLOORS - I Care has to be ta)en in the construction of composite floors !ith profiled dec)ing to pre&ent e5cessi&e 'ponding',  especiall in the case of long spans# The profiled sheetdeflects considerabl re+uiring additional concrete at the centre that ma add to theconcreting cost# Thus( longer spans !ill re+uire propping to eliminate substantialdeflection or need significant +uantities of concrete# Fig# 6 sho!s ponding of the profileddec)# 2.0THE STRUCTURAL ELEMENTS Composite floors !ith profiled dec)ing consists of the follo!ing structural elementsalong !ith in-situ concrete and steel beams1 ã Profiled dec)ing ã Shear connectors ã Reinforcement for shrin)age and temperature stressesConnections bet!een the structural steel elements are generall designed as 7simple8 i#e#not moment resisting# Stud shear connectors are !elded through the sheeting on to thetop flange of the beam# Insulation re+uirements for fire usuall control the slab thic)nessabo&e the profile# Thic)ness &alues bet!een 65  and 120 mm  are sufficient to gi&e a firerating of up to 2  hours# Light!eight concrete is popular( despite its slightl higher initialcost( because of the conse+uent reduction in !eight and enhanced fire-insulation properties# 2.1Profie! s ee# !e$%in& The steel dec) is normall rolled into the desired profile from 0. mm  to 1.5 mm gal&anised coil# It is profiled such that the profile heights are usuall in the range of 3!- 5 mm  and the pitch of corrugations is bet!een 150 mm  and 350 mm # 2enerall( spans of the order of 2.5 m  to 3.5 m  bet!een the beams are chosen and the beams are designed tospan bet!een 6 m  to 12 m # There are t!o !ell-)no!n generic tpes of profiles# ã /o&etail profile Version II23 - 3  Ponding deform#tion Fig. 3 Ponding in profiled decking, due to the weight of concrete  COMPOSITE FLOORS - I ã Trape3oidal profile !ith !eb indentations Version II23 - ' $ircl#r indents &oriont#l indents$he(ron indents)#* +ifferent profiles sed  +o(et#il profile)* +ole std tt oint )c* /pic#l edge det#il )d* Side c#ntile(er with std r#cet )e* /pic#l end c#ntile(er  Fig.  !eck profiles and typical details
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